The present invention relates to a method and apparatus for the correction of drift in the level of a video signal provided by a flying spot telecine, in particular, drift arising from instability or progressive drift in the preliminary stages of telecine equipment.
Such errors may be caused, for example, by variations in the intensity of the output of the cathode ray tube ("CRT") scanning means used in flying spot telecine equipment or by variations in photomultiplier tube sensitivity or amplifier gain. These variations are usually of a relatively long term nature. That is to say they cause the video signal level or the colour balance to drift over a period of minutes or longer.
Attempts have been made to reduce signal drift due to variations in the sensitivity of photomultiplier tubes by placing heating elements adjacent the photomultiplier tubes to maintain them at a constant temperature. While this does reduce signal drift to an extent, the elevated temperatures which result increase the noise contributions to the output signal from the photomultiplier tubes and adjacent amplifiers. Furthermore, the use of heating elements has no effect on drift which results from factors other than temperature.